1. Field of the Invention
This invention relates to a voltage comparator circuit for detecting the amplitude of an input signal or the like, and more particularly to a voltage comparator circuit whose input/output characteristics have hysteresis characteristics.
2. Description of the Related Art
A voltage comparator circuit having hysteresis characteristics is used to detect the amplitude of an input signal to prevent an erroneous operation due to noise included in the input signal. For example, it is disclosed in pp 32-33 of the article "Operation Amplifier and the Analysis thereof by Experiment", written by Hideo Tsunoda, Tokyo Denki University Publishing Department. FIG. 5 shows an example of the conventional voltage comparator circuit having hysteresis characteristics. In the circuit of FIG. 5, input voltage V.sub.IN is applied to inverting input terminal (-) of operation amplifier 51, positive feedback resistor R2 is connected between the output terminal and non-inverting input terminal (+), and resistor R1 is connected between the non-inverting input terminal and reference potential terminal V.sub.GND. In the above conventional voltage comparator circuit, comparison reference voltage V.sub.REF at the non-inverting input terminal (+) can be expressed as ##EQU1## where Vout is an output voltage.
Assume now that the high potential side power source voltage of operation amplifier 51 is V.sub.DD and the lower potential side power source voltage is Vss. In this case, reference potential V.sub.REF.sup.+ set when output voltage Vout is at a high level (V.sub.DD potential) which is given by equation: ##EQU2## and reference potential V.sub.REF.sup.-, set when output voltage Vout is at a low level (Vss potential), is given by equation: ##EQU3##
In this way, the above voltage comparator circuit compares input voltage V.sub.IN with high reference potential V.sub.REF.sup.+ at the time of the rising thereof, and with low reference potential V.sub.REF.sup.- at the time of fall. Therefore, as shown in FIG. 6, the input/output characteristics thereof will have hysteresis characteristics. In this case, the hysteresis width (V.sub.REF.sup.+ -V.sub.REF.sup.-) is given as ##EQU4##
In the above voltage comparator circuit, (V.sub.DD -Vss) in equation (4) will vary according to variations in power source voltages V.sub.DD and Vss, and, as a result, the hysteresis width will vary depending on the power source voltage. Therefore, when the above voltage comparator circuit is applied to an input signal amplitude detection circuit, the detected level will vary depending on the power source voltage. As a result, it is impossible to use the voltage comparator circuit when it is required to detect the amplitude with high precision.
Further, as can be seen from equation (4), the hysteresis width also depends on (R1/R1+R2). Therefore, it is necessary to set the resistance ratio of (R1+R2) to R1 large when the hysteresis width requires to be set small. However, in an integrated circuit, it is difficult to form resistors R1 to R2 so as to attain a large ratio in area of resistor R2 to resistor R1.
When the hysteresis characteristics are determined in order that the hysteresis width components measured from potential V.sub.GND in the higher and lower potential directions are equal to each other (i.e., the hysteresis curve is symmetrical with respect to potential V.sub.GND), potential V.sub.GND then cannot be set to a value other than (V.sub.DD -Vss)/2. Consequently, in this case, potential V.sub.GND is restricted to a certain value and the degree of freedom with regard to setting potential V.sub.GND therefore nullified.
To summarize, in the conventional voltage comparator circuit, the hysteresis width has dependency on the power source voltage, and is difficult to set to a small value. In addition, the range in which potential V.sub.DD can be set is restricted when the hysteresis width is set to be symmetric with respect to reference potential V.sub.GND.